Filtering patient signal also for ventilation artifacts

ABSTRACT

In embodiments, an external medical device is intended to care for a patient. If it receives an input that signifies that ventilation artifact is present in a signal of the patient, it transmits a corrective signal responsive to the received input. In further embodiments, a patient signal is received, which is generated from a patient while the patient is or was receiving chest compressions at a frequency Fc, and also receiving ventilations at frequency Fv. At least one filter mechanism may be applied to the patient signal to substantially remove artifacts at a) frequency Fc, b) a higher harmonic of frequency Fc, and c) a third frequency substantially equaling frequency Fc plus or minus frequency Fv, while substantially passing other frequencies between them. As a result, the patient signal can be cleaner, for diagnosing the patient&#39;s state more accurately.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 15/048,641, filed on Feb. 19, 2016 and issued on Mar. 14, 2017 asU.S. Pat. No. 9,592,012, which in turn is a divisional of U.S. patentapplication Ser. No. 14/930,631, filed on Nov. 2, 2015 and issued onMar. 29, 2016 as U.S. Pat. No. 9,295,427, which in turn is a divisionalof U.S. patent application Ser. No. 14/032,376, filed on Sep. 20, 2013and issued on Dec. 8, 2015 as U.S. Pat. No. 9,204,845, which in turnclaims priority from U.S. Provisional Patent Application Ser. No.61/704,932, filed on Sep. 24, 2012, and which further is aContinuation-In-Part of U.S. patent application Ser. No. 13/676,593,filed on Nov. 14, 2012 and issued on Jul. 21, 2015 as U.S. Pat. No.9,084,545 B2, all commonly assigned herewith.

BACKGROUND

In humans, the heart beats to sustain life. In normal operation, itpumps blood through the various parts of the body. More particularly,the various chamber of the heart contract and expand in a periodic andcoordinated fashion, which causes the blood to be pumped regularly. Morespecifically, the right atrium sends deoxygenated blood into the rightventricle. The right ventricle pumps the blood to the lungs, where itbecomes oxygenated, and from where it returns to the left atrium. Theleft atrium pumps the oxygenated blood to the left ventricle. The leftventricle, then, expels the blood, forcing it to circulate to thevarious parts of the body.

The heart chambers pump because of the heart's electrical controlsystem. More particularly, the sinoatrial (SA) node generates anelectrical impulse, which generates further electrical signals. Thesefurther signals cause the above-described contractions of the variouschambers in the heart, in the correct sequence. The electrical patterncreated by the sinoatrial (SA) node is called a sinus rhythm.

Sometimes, however, the electrical control system of the heartmalfunctions, which can cause the heart to beat irregularly, or not atall. The cardiac rhythm is then generally called an arrhythmia.Arrhythmias may be caused by electrical activity from locations in theheart other than the SA node. Some types of arrhythmia may result ininadequate blood flow, thus reducing the amount of blood pumped to thevarious parts of the body. Some arrhythmias may even result in a SuddenCardiac Arrest (SCA). In a SCA, the heart fails to pump bloodeffectively, and, if not treated, death can occur. In fact, it isestimated that SCA results in more than 250,000 deaths per year in theUnited States alone. Further, a SCA may result from a condition otherthan an arrhythmia.

One type of arrhythmia associated with SCA is known as VentricularFibrillation (VF). VF is a type of malfunction where the ventricles makerapid, uncoordinated movements, instead of the normal contractions. Whenthat happens, the heart does not pump enough blood to deliver enoughoxygen to the vital organs. The person's condition will deterioraterapidly and, if not reversed in time, they will die soon, e.g. withinten minutes.

Ventricular Fibrillation can often be reversed using a life-savingdevice called a defibrillator. A defibrillator, if applied properly, canadminister an electrical shock to the heart. The shock may terminate theVF, thus giving the heart the opportunity to resume pumping blood. If VFis not terminated, the shock may be repeated, often at escalatingenergies.

A challenge with defibrillation is that the electrical shock must beadministered very soon after the onset of VF. There is not much time:the survival rate of persons suffering from VF decreases by about 10%for each minute the administration of a defibrillation shock is delayed.After about 10 minutes the rate of survival for SCA victims averagesless than 2%.

The challenge of defibrillating early after the onset of VF is being metin a number of ways. First, for some people who are considered to be ata higher risk of VF or other heart arrhythmias, an ImplantableCardioverter Defibrillator (ICD) can be implanted surgically. An ICD canmonitor the person's heart, and administer an electrical shock asneeded. As such, an ICD reduces the need to have the higher-risk personbe monitored constantly by medical personnel.

Regardless, VF can occur unpredictably, even to a person who is notconsidered at risk. As such, VF can be experienced by many people wholack the benefit of ICD therapy. When VF occurs to a person who does nothave an ICD, they collapse, because blood flow has stopped. They shouldreceive therapy quickly.

For a VF victim without an ICD, a different type of defibrillator can beused, which is called an external defibrillator. External defibrillatorshave been made portable, so they can be brought to a potential VF victimquickly enough to revive them.

During VF, the person's condition deteriorates, because the blood is notflowing to the brain, heart, lungs, and other organs. Blood flow must berestored, if resuscitation attempts are to be successful.

Cardiopulmonary Resuscitation (CPR) is one method of forcing blood flowin a person experiencing cardiac arrest. In addition, CPR is the primaryrecommended treatment for some patients with some kinds of non-VFcardiac arrest, such as asystole and pulseless electrical activity(PEA). CPR is a combination of techniques that include chestcompressions to force blood circulation, and rescue breathing to forcerespiration.

Properly administered CPR provides oxygenated blood to critical organsof a person in cardiac arrest, thereby minimizing the deterioration thatwould otherwise occur. As such, CPR can be beneficial for personsexperiencing VF, because it slows the deterioration that would otherwiseoccur while a defibrillator is being retrieved. Indeed, for patientswith an extended down-time, survival rates are higher if CPR isadministered prior to defibrillation.

Advanced medical devices can actually coach a rescuer who performs CPR.For example, a medical device can issue instructions, and even prompts,for the rescuer to perform CPR more effectively.

There are also CPR machines, namely mechanical chest compression devicesthat will perform CPR on a patient. One example is the LUCAS® devicesold by Physio-Control, Inc. For the rescue breathing component of CPR,rescuers may perform manual ventilations for the patient, called“breaths”. In addition, there are automatic ventilators that willprovide ventilations for the patient. A problem is that the compressionsand the ventilations may introduce undesirable artifact in the patientsignal, making it harder for machines to diagnose the patient.

BRIEF SUMMARY

The present description gives instances of methods, medical devices,methods of operating such medical devices, systems, and programmedprocessors to control such medical devices for removing chest artifactsfrom the signal of a patient who is receiving chest compressions andventilations.

In embodiments, an external medical device is intended to care for apatient. If it receives an input that signifies that ventilationartifact is present in a signal of the patient, it transmits acorrective signal responsive to the received input. As a result, thepatient signal can be cleaner, either by abating a source of theartifact or by applying a filter to it.

In further embodiments, a patient signal is received, which is generatedfrom a patient while the patient is or was receiving chest compressionsat a frequency Fc, and also receiving ventilations at frequency Fv. Atleast one filter mechanism may be applied to the patient signal tosubstantially remove artifacts at a) frequency Fc, b) a higher harmonicof frequency Fc, and c) a third frequency substantially equalingfrequency Fc plus or minus frequency Fv, while substantially passingother frequencies between them. As a result, the patient signal can becleaner, for diagnosing the patient's state more accurately.

These and other features and advantages of this description will becomemore readily apparent from the following Detailed Description, whichproceeds with reference to the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a scene where an external defibrillator is usedto save the life of a person according to embodiments.

FIG. 2 is a table listing two main types of the external defibrillatorshown in FIG. 1, and who they might be used by.

FIG. 3 is a functional block diagram showing components of an externaldefibrillator, such as the one shown in FIG. 1, which is made accordingto embodiments.

FIG. 4 is a functional block diagram showing monitoring a patient signalaccording to embodiments.

FIG. 5 is a diagram of operating frequencies of machines used in FIG. 4.

FIG. 6 is a time diagram of a patient ECG signal who is receiving chestcompressions and ventilations.

FIG. 7 is a flowchart for illustrating methods according to embodiments.

FIG. 8 is a flowchart for illustrating methods according to embodiments.

FIG. 9 is a graphical illustration of a fast Fourier transform of an ECGsignal of an asystolic patient receiving chest compressions from aconventional mechanical chest compression device.

FIG. 10 is a graphical illustration of a fast Fourier transform of anECG signal from an asystolic patient receiving chest compressions from amechanical chest compression device having precise frequency controlaccording to embodiments.

FIG. 11 is a graphical illustration of the frequency response of a combfilter suitable for removing chest compression artifacts from an ECGsignal according to embodiments.

FIG. 12 is a graphical illustration of the frequency response of aninverse comb filter suitable for removing chest compression artifactsfrom an ECG signal according to embodiments.

FIG. 13 is a diagram showing sample frequencies that have artifacts, andwhich are rejected by filters and filtering according to embodiments.

FIG. 14 is a graphical illustration of an estimated power noise spectrumin a patient signal of patient receiving concurrently chest compressionsand ventilations according to embodiments.

FIG. 15 is a flowchart for illustrating methods according toembodiments.

DETAILED DESCRIPTION

As has been mentioned, the present description is about medical devices,methods of operating such medical devices, systems, and programmedprocessors to control such medical devices for removing chest artifactsfrom the signal of a patient who is receiving chest compressions andventilations.

Embodiments are now described in more detail.

FIG. 1 is a diagram of a defibrillation scene. A person 82 is lying ontheir back. Person 82 could be a patient in a hospital, or someone foundunconscious, and then turned to be on their back. Person 82 isexperiencing a condition in their heart 85, which could be VentricularFibrillation (VF).

A portable external defibrillator 100 has been brought close to person82. At least two defibrillation electrodes 104, 108 are usually providedwith external defibrillator 100, and are sometimes called electrodes104, 108. Electrodes 104, 108 are coupled with external defibrillator100 via respective electrode leads 105, 109. A rescuer (not shown) hasattached electrodes 104, 108 to the skin of person 82. Defibrillator 100is administering, via electrodes 104, 108, a brief, strong electricpulse 111 through the body of person 82. Pulse 111, also known as adefibrillation shock, goes also through heart 85, in an attempt torestart it, for saving the life of person 82.

Defibrillator 100 can be one of different types, each with differentsets of features and capabilities. The set of capabilities ofdefibrillator 100 is determined by planning who would use it, and whattraining they would be likely to have. Examples are now described.

FIG. 2 is a table listing two main types of external defibrillators, andwho they are primarily intended to be used by. A first type ofdefibrillator 100 is generally called a defibrillator-monitor, becauseit is typically formed as a single unit in combination with a patientmonitor. A defibrillator-monitor is sometimes calledmonitor-defibrillator. A defibrillator-monitor is intended to be used bypersons in the medical professions, such as doctors, nurses, paramedics,emergency medical technicians, etc. Such a defibrillator-monitor isintended to be used in a pre-hospital or hospital scenario.

As a defibrillator, the device can be one of different varieties, oreven versatile enough to be able to switch among different modes thatindividually correspond to the varieties. One variety is that of anautomated defibrillator, which can determine whether a shock is neededand, if so, charge to a predetermined energy level and instruct the userto administer the shock. Another variety is that of a manualdefibrillator, where the user determines the need and controlsadministering the shock.

As a patient monitor, the device has features additional to what isminimally needed for mere operation as a defibrillator. These featurescan be for monitoring physiological indicators of a person in anemergency scenario. These physiological indicators are typicallymonitored as signals. For example, these signals can include a person'sfull ECG (electrocardiogram) signals, or impedance between twoelectrodes. Additionally, these signals can be about the person'stemperature, non-invasive blood pressure (NIBP), arterial oxygensaturation/pulse oximetry (SpO2), the concentration or partial pressureof carbon dioxide in the respiratory gases, which is also known ascapnography, and so on. These signals can be further stored and/ortransmitted as patient data.

A second type of external defibrillator 100 is generally called an AED,which stands for “Automated External Defibrillator”. An AED typicallymakes the shock/no shock determination by itself, automatically. Indeed,it can sense enough physiological conditions of the person 82 via onlythe shown defibrillation electrodes 104, 108 of FIG. 1. In its presentembodiments, an AED can either administer the shock automatically, orinstruct the user to do so, e.g. by pushing a button. Being of a muchsimpler construction, an AED typically costs much less than adefibrillator-monitor. As such, it makes sense for a hospital, forexample, to deploy AEDs at its various floors, in case the moreexpensive defibrillator-monitor is more critically being deployed at anIntensive Care Unit, and so on.

AEDs, however, can also be used by people who are not in the medicalprofession. More particularly, an AED can be used by many professionalfirst responders, such as policemen, firemen, etc. Even a person withonly first-aid training can use one. And AEDs increasingly can supplyinstructions to whoever is using them.

AEDs are thus particularly useful, because it is so critical to respondquickly, when a person suffers from VF. Indeed, the people who willfirst reach the VF sufferer may not be in the medical professions.

Increasing awareness has resulted in AEDs being deployed in public orsemi-public spaces, so that even a member of the public can use one, ifthey have obtained first aid and CPR/AED training on their owninitiative. This way, defibrillation can be administered soon enoughafter the onset of VF, to hopefully be effective in rescuing the person.

There are additional types of external defibrillators, which are notlisted in FIG. 2. For example, a hybrid defibrillator can have aspectsof an AED, and also of a defibrillator-monitor. A usual such aspect isadditional ECG monitoring capability.

FIG. 3 is a diagram showing components of an external defibrillator 300made according to embodiments. These components can be, for example, inexternal defibrillator 100 of FIG. 1. Plus, these components of FIG. 3can be provided in a housing 301, which is also known as casing 301.

External defibrillator 300 is intended for use by a user 380, who wouldbe the rescuer. Defibrillator 300 typically includes a defibrillationport 310, such as a socket in housing 301. Defibrillation port 310includes nodes 314, 318. Defibrillation electrodes 304, 308, which canbe similar to electrodes 104, 108, can be plugged in defibrillation port310, so as to make electrical contact with nodes 314, 318, respectively.It is also possible that electrodes can be connected continuously todefibrillation port 310, etc. Either way, defibrillation port 310 can beused for guiding via electrodes to person 82 an electrical charge thathas been stored in defibrillator 300, as will be seen later in thisdocument.

If defibrillator 300 is actually a defibrillator-monitor, as wasdescribed with reference to FIG. 2, then it will typically also have anECG port 319 in housing 301, for plugging in ECG leads 309. ECG leads309 can help sense an ECG signal, e.g. a 12-lead signal, or from adifferent number of leads. Moreover, a defibrillator-monitor could haveadditional ports (not shown), and an other component 325 structured tofilter the ECG signal, e.g., apply at least one filter to the signal soas to remove chest compression artifacts resulting from chestcompressions being delivered to the person 82.

Defibrillator 300 also includes a measurement circuit 320. Measurementcircuit 320 receives physiological signals from ECG port 319, and alsofrom other ports, if provided. These physiological signals are sensed,and information about them is rendered by circuit 320 as data, or othersignals, etc.

If defibrillator 300 is actually an AED, it may lack ECG port 319.Measurement circuit 320 can obtain physiological signals through nodes314, 318 instead, when defibrillation electrodes 304, 308 are attachedto person 82. In these cases, a person's ECG signal can be sensed as avoltage difference between electrodes 304, 308. Plus, impedance betweenelectrodes 304, 308 can be sensed for detecting, among other things,whether these electrodes 304, 308 have been inadvertently disconnectedfrom the person.

Defibrillator 300 also includes a processor 330. Processor 330 may beimplemented in any number of ways, for causing actions and operations tobe performed. Such ways include, by way of example and not oflimitation, digital and/or analog processors such as microprocessors anddigital-signal processors (DSPs); controllers such as microcontrollers;software running in a machine; programmable circuits such as FieldProgrammable Gate Arrays (FPGAs), Field-Programmable Analog Arrays(FPAAs), Programmable Logic Devices (PLDs), Application SpecificIntegrated Circuits (ASICs), any combination of one or more of these,and so on.

Processor 330 can be considered to have a number of modules. One suchmodule can be a detection module 332, which senses outputs ofmeasurement circuit 320. Detection module 332 can include a VF detector.Thus, the person's sensed ECG can be used to determine whether theperson is experiencing VF.

Another such module in processor 330 can be an advice module 334, whicharrives at advice based on outputs of detection module 332. Advicemodule 334 can include a Shock Advisory Algorithm, implement decisionrules, and so on. The advice can be to shock, to not shock, toadminister other forms of therapy, and so on. If the advice is to shock,some external defibrillator embodiments merely report that to the user,and prompt them to do it. Other embodiments further execute the advice,by administering the shock. If the advice is to administer CPR,defibrillator 300 may further issue prompts for it, and so on.

Processor 330 can include additional modules, such as module 336, forother functions. In addition, if other component 325 is indeed provided,it may be operated in part by processor 330, etc.

Defibrillator 300 optionally further includes a memory 338, which canwork together with processor 330. Memory 338 may be implemented in anynumber of ways. Such ways include, by way of example and not oflimitation, nonvolatile memories (NVM), read-only memories (ROM), randomaccess memories (RAM), any combination of these, and so on. Memory 338,if provided, can include programs for processor 330, and so on. Theprograms can be operational for the inherent needs of processor 330, andcan also include protocols and ways that decisions can be made by advicemodule 334. In addition, memory 338 can store prompts for user 380, etc.Moreover, memory 338 can store patient data.

Defibrillator 300 may also include a power source 340. To enableportability of defibrillator 300, power source 340 typically includes abattery. Such a battery is typically implemented as a battery pack,which can be rechargeable or not. Sometimes, a combination is used, ofrechargeable and non-rechargeable battery packs. Other embodiments ofpower source 340 can include AC power override, for where AC power willbe available, and so on. In some embodiments, power source 340 iscontrolled by processor 330.

Defibrillator 300 additionally includes an energy storage module 350.Module 350 is where some electrical energy is stored, when preparing itfor sudden discharge to administer a shock. Module 350 can be chargedfrom power source 340 to the right amount of energy, as controlled byprocessor 330. In typical implementations, module 350 includes one ormore capacitors 352, and so on.

Defibrillator 300 moreover includes a discharge circuit 355. Circuit 355can be controlled to permit the energy stored in module 350 to bedischarged to nodes 314, 318, and thus also to defibrillation electrodes304, 308. Circuit 355 can include one or more switches 357. Those can bemade in a number of ways, such as by an H-bridge, and so on.

Defibrillator 300 further includes a user interface 370 for user 380.User interface 370 can be made in any number of ways. For example,interface 370 may include a screen, to display what is detected andmeasured, provide visual feedback to the rescuer for their resuscitationattempts, and so on. Interface 370 may also include a speaker, to issuevoice prompts, etc. Interface 370 may additionally include variouscontrols, such as pushbuttons, keyboards, and so on. In addition,discharge circuit 355 can be controlled by processor 330, or directly byuser 380 via user interface 370, and so on.

Defibrillator 300 can optionally include other components. For example,a communication module 390 may be provided for communicating with othermachines. Such communication can be performed wirelessly, or via wire,or by infrared communication, and so on. This way, data can becommunicated, such as patient data, incident information, therapyattempted, CPR performance, and so on.

A feature of a defibrillator can be CPR-prompting. Prompts are issued tothe user, visually or by sound, so that the user can administer CPR.Examples are taught in U.S. Pat. Nos. 6,334,070 and 6,356,785.

FIG. 4 is a functional block diagram showing monitoring a patient signalaccording to embodiments. A mechanical chest compression device 485delivers chest compressions 495 to patient 482. The compressions are ata nominal frequency Fc. A sample value of Fc is about 100 compressionsper minute (1.667 Hz). Mechanical chest compression devices may or maynot have substantially precise frequency control. In addition, aventilator 486 delivers ventilations 496 to patient 482 at a nominalfrequency Fv. Ventilator 486 may be an electrical/mechanical device, orit could be a person operating a setup such as a bag-valve mask or aself-inflating bag attached to an endotracheal tube. A sample value ofFv is about 10 ventilations per minute (0.167 Hz).

FIG. 5 is a diagram of the frequencies of possible machines 485, 486that can be used in FIG. 4. In the example of FIG. 5, the machines haveprecise frequency control. A frequency component 595 is contributed atFc from compressions 495, and a frequency component 596 is contributedat Fv from ventilations 496.

Returning to FIG. 4, an external device 400 is provided, which ispreferably also a medical device intended to care for a patient. Device400 has a housing 401, and an ECG port 419 that can receive a patientsignal, such as an ECG signal or an impedance signal. As mentionedabove, the patient signal may have artifacts due to compressions 495 andventilations 496 that patient 482 is receiving.

Device 400 also has a display 470 in connection with the housing 401,and possibly other output devices, such as a speaker, and so on. Display470 is for use by user/rescuer 480. Device 400 also has, within housing401, a processor 430 that could be similar to processor 330. Device 400optionally also has, within housing 401, a memory 438. Memory 438 couldbe a non-transitory storage medium that stores programs for execution byprocessor 430. Processor 430 may be further configured to cause thedisplay 470 to visually present, to user 480, a version of the patientsignal. The version could be as the signal is received, or as has beenfiltered as described below. Alternatively, or in addition, theprocessor 430 may be configured to cause an optional printer 439 toprint out the version of the patient signal.

In some embodiments, device 400 also has, within housing 401, a filtermechanism 425. In this example, filter mechanism 425 is shown asseparate from processor 430, although equivalently processor 430 canperform the functions of filter mechanism 425 according to embodiments.

FIG. 6 is a time diagram of an ECG signal of a patient who is receivingchest compressions and ventilations, such as patient 482 of FIG. 4. Thehorizontal axis represents time, and the vertical axis represents signalamplitude. In the example of FIG. 6, the patient is in ventricularfibrillation (“VF”), but the ECG signal is obscured by artifact.Compression artifact shows up as roughly sine-wave components 695 thatoccur slightly more than once per second. Ventilation artifact shows upas modulations 696 of the compression artifact. In this diagram, theventilation artifact has a period of about 6 seconds.

FIG. 7 shows a flowchart 700 for describing methods according toembodiments. The methods of flowchart 700 may also be user 480 who caresfor patient 482.

According to an operation 710, it is detected that ventilation artifactis present in a signal of the patient. Detecting can be performed in anumber of ways. In some embodiments, detecting is performed byinspecting a visual representation of the patient signal, such as thatof FIG. 6. Characteristics of ventilation artifact may be recognized. Inother embodiments, detecting is performed by an automatic analysis ofthe patient signal. For example, the analyzed patient signal is one ofan ECG signal and an impedance signal of the patient. In otherembodiments, detecting is performed by viewing a correlation of anairway signal of the patient, with another signal of the patient. Theairway signal can be a capnography signal, an airway pressure signal, oran air flow signal. The other signal can be the ECG signal or theimpedance signal. The correlation may reveal the presence of theventilation artifact in the patient signal.

According to another operation 720, a corrective action is taken,responsive to so detecting at operation 710. The corrective action canbe taken in a number of ways. In some embodiments, the corrective actionincludes discontinuing an operation of a ventilator that presumptivelyhas caused the ventilation artifact. The discontinuing can beindefinite, or merely the beginning of a pause, after which theventilator operation is resumed. In the example of FIG. 4, user 482would discontinue operation of ventilator 486. In other embodiments, thecorrective action includes discontinuing manual ventilations, such asfrom one of the rescuers. In other embodiments, the corrective actionincludes causing an augmented filter to be applied to the patientsignal, so that the ventilation artifact is removed. In the example ofFIG. 4, user 482 might enter the appropriate inputs for filter mechanism425 or for another filter to be applied to the patient signal.

The above-mentioned devices and/or systems perform functions, processesand/or methods, as described in this document. The functions, processesand/or methods may be implemented by one or more devices that includelogic circuitry. Such a device can be alternately called a computer, adevice, and so on. It may be a standalone device or computer, such as ageneral purpose computer, or part of a device that has one or moreadditional functions. The logic circuitry may include a processor thatmay be programmable for a general purpose, or dedicated, such as amicrocontroller, a microprocessor, a Digital Signal Processor (DSP),etc. The logic circuitry may also include storage media, such as amemory. Such media include but are not limited to volatile memory,non-volatile memory (NVM), read only memory (ROM); random access memory(RAM); magnetic disk storage media; optical storage media; smart cards,flash memory devices, etc. Any one of these storage media could be anon-transitory computer-readable medium. These storage media,individually or in combination with others, can have stored thereonprograms that the processor may be able to read, and execute. Moreparticularly, the programs can include instructions in the form of code,which the processor may be able to execute upon reading. Executing isperformed by physical manipulations of physical quantities, and mayresult in the functions, processes and/or methods to be performed. Inaddition, these storage media may store data.

Moreover, methods and algorithms are described below. These methods andalgorithms are not necessarily inherently associated with any particularlogic device or other apparatus. Rather, they are advantageouslyimplemented by programs for use by a computing machine, such as ageneral-purpose computer, a special purpose computer, a microprocessor,etc.

Often, for the sake of convenience only, it is preferred to implementand describe a program as various interconnected distinct softwaremodules or features, individually and collectively also known assoftware. This is not necessary, however, and there may be cases wheremodules are equivalently aggregated into a single program, even withunclear boundaries. In some instances, software is combined withhardware, in a mix called firmware.

This detailed description includes flowcharts, display images,algorithms, and symbolic representations of program operations within atleast one computer readable medium. An economy is achieved in that asingle set of flowcharts is used to describe both programs, and alsomethods. So, while flowcharts described methods in terms of boxes, theyalso concurrently describe programs. A method is now described.

FIG. 8 shows a flowchart 800 for describing methods according toembodiments. The methods of flowchart 800 may also be practiced byembodiments described above, such as medical device 400.

According to an operation 810, an input is received, which signifiesthat ventilation artifact is present in a signal of the patient. Theinput may be received in any number of ways. In some embodiments, theinput is received from a user, such as user 480. In some embodiments, acorrelation is performed of an airway signal of the patient with anothersignal of the patient. The input can be received responsive to anoutcome of the correlation. In other embodiments the input may bereceived by a frequency analysis of a patient signal such as the ECGsignal or impedance waveform. In yet other embodiments the input by bereceived by a time-domain analysis of the ECG signal. As seen in FIG. 6amplitude modulation of the ECG signal at the ventilation frequency ischaracteristic of ventilation artifact.

According to another operation 820, a corrective signal is transmittedresponsive to the received input. The corrective signal can be taken ina number of ways. In some embodiments, the corrective signal causes aprompt to be output to a user, such as user 480. The prompt can beaudible or visible, and can be to the effect of pausing or discontinuingan operation of a ventilator that presumptively has caused theventilation artifact, such as ventilator 486. The prompt may positivelyindicate either the presence or absence of ventilation artifact.Indicating the absence of ventilation artifact is important because ittells the operator that they may be able to trust the results of a combfilter to remove compression artifact. In some embodiments, thecorrective signal can be transmitted to a ventilator whose operationpresumptively has caused the ventilation artifact, such as ventilator486. The corrective signal can thus cause the ventilator operation to bediscontinued. In other embodiments, the corrective signal includescausing an augmented filter to be applied to the patient signal, so thatthe ventilation artifact is removed. In the example of FIG. 4, thecorrective signal might cause filter mechanism 425 or another filter tobe applied to the patient signal.

According to another, optional operation, a record can be caused to becreated. The record can be about the input, the corrective signal, andso on. The created record can be added to the patient record.

Filtering is now described in more detail. Filtering may include usingfilter mechanism 425 and/or one or more augmented filters, as alreadymentioned above.

Filtering is intended to be applied to a received patient signal, so asto result in a filtered signal. Compared to the received patient signal,the filtered signal will lack artifacts that have been removed by thefiltering. As seen in FIG. 5, the source of the artifacts is component595 from the chest compressions at frequency Fc, and component 596 fromthe ventilations at frequency Fv.

In certain embodiments, filter mechanism 425 includes a comb filter. Thecomb filter may be non-adaptive. In other embodiments, filter mechanism425 includes a plurality of notch filters. Each of the notch filters maybe non-adaptive. One having ordinary skill in the art will readilyrecognize that various other filter mechanisms may be used in additionto, or in place of, a comb filter or notch filters.

Certain conventional CPR artifact filters may be adaptive in nature. Asused herein, an adaptive filter generally refers to a filter whosetransfer function is dependent on the patient signal. An adaptive filtermay adjust its filter coefficients, center frequency, roll-off, notchwidth, Q or other characteristic based on the patient signal. Incontrast, non-adaptive filters according to embodiments generally usepredetermined coefficients that may precisely set the transfer functionindependent of the patient signal.

It is possible that a device incorporating this invention may includemultiple non-adaptive filters. The appropriate filter may be selectedbased on patient signal characteristics, such as the frequency contentof the ECG signal or impedance signal. Alternatively, the appropriatefilter may be selected by communication with the mechanical chestcompression device, or through a user input selection.

As for removal of artifacts, some drawings are now presented initiallyto show their nature. The first few drawings address only the artifactdue to the chest compressions. Then the artifact due to the addedventilation will also be addressed.

FIG. 9 is a graphical illustration of a fast Fourier transform of an ECGsignal of an asystolic patient who is receiving only chest compressions.The compressions are from a machine that does not have precise frequencycontrol. As can be seen from the illustrated example, the ECG signalgenerally contains only artifacts, because the patient has no activecardiac signal. Multiple spectral peaks are evident, with thefundamental frequency component 901 of the chest compressions appearingat 1.67 Hz, and other frequency components 902, 903, . . . , 912representing harmonic frequencies. That, even though the source of theartifacts is a component 595 at a single frequency Fc, here 1.67 Hz.Artifact from ventilations 496 would also appear in components at afundamental frequency Fv and higher harmonics.

The width of the spectral peaks in FIG. 9 varies from approximately 0.15Hz at the fundamental frequency, up to approximately 0.5 Hz for the6^(th) harmonic (10 Hz). At first sight, removing the CPR artifact fromthe illustrated signal would necessarily remove much of the cardiacsignal, due to the requirement of a relatively wide filter, and wouldthus cause distortion that impacts adversely the patient signal that isto be extracted.

Signals corresponding to manual CPR or conventional mechanical CPRdevices generally have only broad spectral peaks, and the locations ofsuch peaks are typically not precisely controlled. The fundamentalfrequency may vary from one device to another, or from one applicationto another. For example, the fundamental frequency may vary from 1.4 Hzto 1.7 Hz. Such variation generally prevents application of anon-adaptive filter, e.g., a comb filter, with a narrow stop band.

Conventional CPR artifact filters have been unsuccessful at removing CPRartifacts, in part, because they typically focus on removing thefundamental frequency while paying little, if any, attention to theharmonic frequencies. In the example illustrated by FIG. 9, component912 of the 12^(th) harmonic is only about 11 dB down from component 901of the fundamental frequency. In order to produce a clean ECG signal,CPR artifacts usually need to be attenuated by at least 20 dB, andpossibly as much as 40 dB. In order to clean up the signal, frequenciesup to at least the 12^(th) harmonic must typically be removed.

FIG. 10 is a graphical illustration of a fast Fourier transform of anECG signal from an asystolic patient who is receiving only chestcompressions according to embodiments. The compressions are from amachine that has precise frequency control. The spectral peaks of theartifacts generated by this device are typically very narrow, e.g., lessthan 0.1 Hz wide. This narrow spectral content enables the ECG signal tobe separated from chest compression artifact. As with the signal of FIG.9, beyond a component 1001 of the fundamental frequency, multiplecomponents 1002, 1003, . . . , 1011 of frequency harmonics are presentin the signal of FIG. 10. It will be noted that the 5^(th) harmonic isless than 20 dB down, and the 11^(th) harmonic is less than 40 dB down.In order to clean up the signal, harmonics up to at least the 5^(th)harmonic, and possibly as high as the 11^(th) harmonic, should beremoved.

FIG. 11 is a graphical illustration of the frequency response of a combfilter, which attenuates bands 1101, 1102, . . . , 1006. A comb filterintrinsically removes the component at a fundamental frequency, and atall the harmonics. As such, the comb filter of FIG. 11 would beapplicable for removing artifacts from compressions 495, or ventilations496.

The comb filter can be a high-Q comb filter, e.g., Q=16, suitable forremoving chest compression artifacts from an ECG signal according toembodiments. If the Q is set relatively high, e.g. 16, the filter willsurgically remove the artifact frequencies and leave the otherfrequencies relatively untouched.

In general, high-Q filters are more frequency-selective than low-Qfilters. For example, a comb filter having Q=16 will generally have a 3dB notch width of about 0.1 Hz, whereas a comb filter having Q=4 willtypically have a 3 dB notch width of about 0.5 Hz. A filter having Q=2has approximately a 3 dB notch width of about 1 Hz and usually removesalmost as much of the signal as it retains. A lower-Q filter willgenerally remove more artifacts from a signal than a high-Q filter butwill also remove more of the signal itself. In addition, a low-Q filtertends to produce more ringing, which often provides additionaldistortion.

In order to effectively remove CPR artifacts resulting from applicationof a conventional chest compression device, a very low-Q filter ispreferable. Assuming that at least 20 dB of attenuation is needed, evena filter having Q=2 would generally not be effective in removing theartifact from the signal due to the spectral peaks of the artifact beingtoo tall and too broad.

Because the spectral content of a mechanical CPR device according toembodiments is generally extremely narrow, a high-Q filter may be usedto remove the compression artifact and retain the cardiac ECG signalwith little distortion. Because a mechanical CPR device according toembodiments generally produces compressions at a precisely knownfrequency, the artifact may be filtered using a non-adaptive filter.Combining these two aspects (narrow frequency content and precisefrequency control) according to embodiments may thus enable a high-Qcomb filter to be used as an effective filter for removing CPR artifactsfrom the patient signal.

The following is a Z transform of a suitable comb filter:

${H(z)} = \frac{a\left( {z^{- 1} - z^{- n}} \right)}{1 - {bz}^{- n}}$where “a” is a gain constant, “b” sets the filter Q, and “n” is aninteger that sets the notch frequencies. The Q of this filter may be setby a single coefficient, the constant “b.” For example, b=0.82 for a Qof 16. The value of “n” and the sample frequency may be set to locationsof the comb notch frequencies. In situations where n=75 and the samplerate is 125 Hz, for example, the notch frequencies would be 1 2/3 Hz, 31/3 Hz, 5.0 Hz, etc.

A comb filter generally introduces very little signal delay. The signalis typically delayed by only one sample. For example, at 125 Hz, thedelay would be only 8 milliseconds. From a user's standpoint, this delayis imperceptible. This is in contrast to certain filter structures, suchas finite impulse response (FIR) filters, that can delay the signal by asecond or more. Such a delay could lead to a misalignment between thefiltered ECG and other signals, such as the unfiltered ECG or aninvasive blood pressure waveform, which could be confusing to the user.Alternatively or in addition thereto, a collection of narrow notchfilters, e.g., one filter for the fundamental frequency and one forevery harmonic that needs to be removed, may be used. This small delaymay make a comb filter particularly suitable for an ECG display, inwhich signal delays or misalignment with other monitoring parameters maybe objectionable.

FIG. 12 is a graphical illustration of the frequency response of aninverse comb filter, which may be suitable for detecting chestcompression artifacts from an ECG signal according to embodiments. Aninverse comb filter is generally similar to a comb filter except that itpasses the comb frequencies instead of rejecting them. Such an inversecomb filter may be particularly suitable for detection of mechanicalcompressions delivered at certain rates, e.g., 100 compressions/minute.

FIG. 13 is a diagram showing, as heavy dark lines, sample frequenciesthat have artifacts, and which are rejected by filters, filtermechanisms, and filtering according to embodiments. In the diagram ofFIG. 13, there is no meaning to the vertical dimension. Rather, similarfrequency components are shown having equal heights for clarity. Inaddition, the diagram of FIG. 13 is somewhat idealized forcomprehension, in that each frequency component is shown as havingpractically no width.

The components include a fundamental frequency component 1396 atfrequency Fv, and also higher harmonics of Fv. Only five such higherharmonics are shown. The components also include a fundamental frequencycomponent 1395 at frequency Fc, and also higher harmonics of Fc. Onlyone such higher harmonic is shown. Typically, the values frequencies Fvand Fc would have a ratio of 1:10. In FIG. 13, however, the ratio isshown artificially as only 1:3.25 for clarity of the mixing that is nowdescribed.

Artifacts may exist also in third frequencies such as Fc+Fv, and Fc−Fv,and even harmonics of those. In other words, frequencies Fc and Fv mix,and create artifact in the patient signal also in these thirdfrequencies. In FIG. 13, a component 1397 is shown at a third frequencyFc−Fv, and a component 1398 is shown at another third frequency Fc+Fv.Mixing happens when compressions 495 and ventilations 496 are receivedconcurrently by patient 482. Artifacts are an example of noise.

Filters and filtering according to the invention reject, do not pass, orfilter out at least some of the above mentioned frequencies. However,they admit or pass other frequencies between them, such as a component1399.

For example, the noise spectrum resulting from mixing, and which wouldhave to be filtered, may include filtering 0.167 Hz (plus harmonics),1.667 Hz (plus harmonics), as well as 1.667 Hz+0.167 Hz=1.834 Hz and1.667 Hz−0.167 Hz=1.5 Hz. The same thing happens for every harmonic of1.667 Hz and 0.167 Hz.

FIG. 14 is a graphical illustration of an estimated power noise spectrumin a patient signal of patient receiving concurrently chest compressionsand ventilations according to embodiments. The horizontal axis indicatesfrequency in Hz. The vertical axis indicates power/frequency in dB/Hz.

In FIG. 14, Fc and Fv have values as in the example above, plus eachcomponent has a width, unlike in the idealized diagram of FIG. 13. Thespectrum of FIG. 14 is strikingly different from that of FIG. 10. Afilter that rejects the artifact noise frequencies of FIG. 10 will notsucceed in rejecting the artifact noise frequencies of FIG. 14. Thisproblem tends to occur when ventilation artifact mixes with compressionartifact, such as when ventilations are received concurrently withcompressions. Compression artifact alone can be removed with a combfilter that covers only one fundamental and its harmonics. However, whenventilation artifact is mixed with the compression artifact, the combfilter for the compression artifact alone may not be enough. Ventilationartifact alone generally falls outside the ECG frequency band, and thuscan be removed using standard filtering techniques.

It is desirable to precisely control the ventilation, so that itproduces artifacts of known frequencies, as was indicated in FIG. 13.The artifact from a hand-ventilated patient is hard to remove becausethe ventilation rate is not precisely controlled. If the ventilationrate was precisely controlled and it was known in advance, it ispossible that the artifact could be removed with a comb filter, similarto one used for ECG alone.

It is important that the ventilation rate be chosen to work well withthe monitor, and that the monitor knows the ventilation rate. If theventilation rate were set to 0.167 Hz, then a 750 tap comb filter wouldremove the ventilation artifact from the ECG signal (assuming a 125 Hzsample rate). In fact, it would remove the 0.167 Hz ventilationfundamental and all of its harmonics (i.e. 0.334 Hz, 0.50 Hz, 0.668 Hz,etc.). While a 750 tap comb filter may notch out many frequencies (i.e.all multiples of 0.167 Hz) within the ECG pass band, it may have anundesirable impact on the ECG.

A somewhat less invasive technique would be to use a set of notchfilters chosen to remove the specific frequencies of interest. The mainfrequencies of interest are those at multiples of the compressionfrequency Fc+/− the ventilation frequency Fv. For example, with acompression frequency of 1.667 Hz and a ventilation frequency of 0.167Hz, one might choose to notch out 1.5 Hz, 1.834 Hz, 3.167 Hz, 3.5 Hz andso forth.

FIG. 15 shows a flowchart 1500 for describing methods according toembodiments. The methods of flowchart 1500 may also be practiced byembodiments described above, such as device 400, processor 430, amonitor defibrillator, a computer performing post-event review, and soon.

According to an operation 1510, a patient signal is received. Thepatient signal can be, for example, an ECG signal, an impedance signal,and so on. The patient signal may be received from a patient who isreceiving chest compressions at a frequency Fc from a mechanical chestcompression device, and also receiving ventilations at frequency Fv froma ventilator.

According to another, optional operation 1520, a value of frequency Fcis input. This may be accomplished in a number of ways, according toembodiments. An identification of the mechanical chest compressiondevice may be inputted, by the user or by sensing the device, or bytransmission from the device, such as wireless transmission. Then thefrequency Fc can be looked up from the identification of the mechanicalchest compression device. Or an indication may be received about thefrequency Fc, such as from a user via an interface, or by transmissionfrom the mechanical chest compression device. Or the chest compressionscan be detected and analyzed, and the frequency Fc can be input from theanalysis.

According to another, optional operation 1530, a value of frequency Fvis input. This may be accomplished in a number of ways, according toembodiments. An identification of the ventilator may be inputted, by theuser or by sensing the ventilator, or by transmission from theventilator, such as wireless transmission. Then the frequency Fv can belooked up from the identification of the ventilator. Or an indicationmay be received about the frequency Fv, such as from a user via aninterface, or by transmission from the ventilator. Or the ventilationscan be detected and analyzed, and the frequency Fv can be input from theanalysis.

According to another operation 1540, at least one filter mechanism maybe applied to the patient signal. The filter mechanism may be applied inany way, such as automatically, responsive to an input by a user, and soon. The receiving of the patient signal and applying the filtermechanism can be performed by a device in the field or afterwards, aspart of a post-event review.

The result of applying the filter mechanism may be that artifacts willbe substantially removed from the patient signal, at a) the frequencyFc, b) a higher harmonic of the frequency Fc, and c) a third frequencysubstantially equaling the frequency Fc plus or minus the frequency Fv,such as component 1397 or 1398. In addition, the result will be that thefilter mechanism will substantially pass other frequencies, such ascomponent 1399. These other frequencies can be between the frequency Fcand the higher harmonic, and between the frequency Fc and the thirdfrequency.

The above mentioned frequency components are at a minimum. Preferably,the filter mechanism will substantially reject more frequencies, such asat least two more frequencies that are additional higher harmonics ofthe frequency Fc.

According to another, optional operation 1550, the patient signal isvisually presented to a user, via a display that is part of a userinterface. The displayed patient signal could be a version with theartifacts substantially removed.

According to another, optional operation 1560, a version of the patientsignal is stored in a memory. The version could be with the artifactsremoved, or not removed.

According to another, optional operation, an impedance signal of thepatient is monitored. Return of spontaneous circulation (ROSC) isdetected by applying a signal-averaging filter to the impedance signal.

In the methods described above, each operation can be performed as anaffirmative step of doing, or causing to happen, what is written thatcan take place. Such doing or causing to happen can be by the wholesystem or device, or just one or more components of it. In addition, theorder of operations is not constrained to what is shown, and differentorders may be possible according to different embodiments. Moreover, incertain embodiments, new operations may be added, or individualoperations may be modified or deleted. The added operations can be, forexample, from what is mentioned while primarily describing a differentsystem, device or method.

Another strategy is to avoid creating ventilation artifact in the firstplace by using a ventilation strategy such as continuous oxygeninsufflation. Continuous oxygen insufflation is a method of providingoxygen to the patient that does not move a large volume of air in andout of the lungs. Instead, a small amount of oxygen is fed in to theairway on a continuous basis. Because the oxygen flow is not periodic itwill not mix with the ECG signal.

This description includes one or more examples, but that does not limithow the invention may be practiced. Indeed, examples or embodiments ofthe invention may be practiced according to what is described, or yetdifferently, and also in conjunction with other present or futuretechnologies.

A person skilled in the art will be able to practice the presentinvention in view of this description, which is to be taken as a whole.Details have been included to provide a thorough understanding. In otherinstances, well-known aspects have not been described, in order to notobscure unnecessarily the present invention.

Other embodiments include combinations and sub-combinations of featuresdescribed herein, including for example, embodiments that are equivalentto: providing or applying a feature in a different order than in adescribed embodiment, extracting an individual feature from oneembodiment and inserting such feature into another embodiment; removingone or more features from an embodiment; or both removing a feature froman embodiment and adding a feature extracted from another embodiment,while providing the advantages of the features incorporated in suchcombinations and sub-combinations.

The following claims define certain combinations and subcombinations ofelements, features and steps or operations, which are regarded as noveland non-obvious. Additional claims for other such combinations andsubcombinations may be presented in this or a related document.

What is claimed is:
 1. An external device for use when a patient isreceiving ventilations from a ventilator that is being operated by arescuer, comprising: a housing; and a processor within the housingconfigured to: perform a correlation of a sensed patient impedancesignal with an airway signal of the patient; receive, responsive to theperformed correlation, an input signifying that a ventilation artifactis present in the sensed patient impedance signal, the ventilationartifact possibly due to the ventilations from the ventilator; and causeto be transmitted a corrective signal responsive to the received input;and a user interface configured to output, responsive to the correctivesignal, a prompt to the rescuer.
 2. The device of claim 1, in which theprompt indicates the presence of a ventilation artifact.
 3. The deviceof claim 1, in which the prompt is to the effect that the rescuer shoulddiscontinue operating the ventilator that presumptively has caused theventilation artifact.
 4. The device of claim 1, in which the airwaysignal is a capnography signal, an airway pressure signal, or an airflowsignal of the patient.
 5. The device of claim 1, in which the processoris further configured to: cause a record to be created about one of theinput and the corrective signal.
 6. A non-transitory computer-readablestorage medium storing one or more programs which, when executed by atleast one external device that is intended for use when a patient isreceiving ventilations from a ventilator that is being operated by arescuer, the external device including a processor and a user interface,the one or more programs result in: performing a correlation of a sensedpatient impedance signal with an airway signal of the patient;receiving, responsive to the performed correlation, an input signifyingthat a ventilation artifact is present in the sensed patient impedancesignal, the ventilation artifact possibly due to the ventilations fromthe ventilator; causing to be transmitted a corrective signal responsiveto the received input; and causing to be output, responsive to thecorrective signal, a prompt to the rescuer.
 7. The medium of claim 6, inwhich the prompt indicates the presence of a ventilation artifact. 8.The medium of claim 6, in which the prompt is to the effect that therescuer should discontinue operating the ventilator that presumptivelyhas caused the ventilation artifact.
 9. The medium of claim 6, in whichthe airway signal is a capnography signal, an airway pressure signal, oran airflow signal of the patient.
 10. The medium of claim 6, in whichexecuting the one or more programs further results in: causing a recordto be created about one of the input and the corrective signal.
 11. Amethod for an external medical device intended for use when a patient isreceiving ventilations from a ventilator that is being operated by arescuer, the method comprising: performing a correlation of a sensedpatient impedance signal with an airway signal of the patient;receiving, responsive to the performed correlation, an input signifyingthat a ventilation artifact is present in the sensed patient impedancesignal, the ventilation artifact possibly due to the ventilations fromthe ventilator; transmitting a corrective signal responsive to thereceived input; and outputting, responsive to the corrective signal, aprompt to the rescuer.
 12. The method of claim 11, in which the promptindicates the presence of a ventilation artifact.
 13. The method ofclaim 11, in which the prompt is to the effect that the rescuer shoulddiscontinue operating the ventilator that presumptively has caused theventilation artifact.
 14. The method of claim 11, in which the airwaysignal is a capnography signal, an airway pressure signal, or an airflowsignal of the patient.
 15. The method of claim 11, further comprising:causing a record to be created about one of the input and the correctivesignal.